Organic vapor deposition process for corrosion protection of metal substrates

ABSTRACT

A process of organic vapor deposition for significantly improved corrosion protection of metal. The process involves deposition of a thin film by polymerization of certain precursors onto the surface of a metal phosphate treated metal substrate (15). A primer or topcoat is subseqently applied over the metal substrate.

FIELD OF THE INVENTION

This invention relates to corrosion protection of metal substrates byorganic vapor deposition of a thin polymeric film preliminary to aprimer or topcoat. The invention is especially applicable to thecorrosion protection of galvanized steel substrates used in automotiveproduction.

BACKGROUND

The corrosion protection of steel substrates is important for manyindustries, including the automotive and steel industries. Currently,the most common methods of corrosion protection of steel substratesinclude galvanizing, application of zinc phosphate, electrodeposition oforganics, conventional spray or dip priming, oil coating, andcombinations thereof. However, for high performance uses in theautomotive industry, such methods are associated with the followingproblems: (1) pollution in the form of volatile organic compounds (VOC),(2) excessive waste disposal, (3) inadequate coverage of recessed areas,and (4) inadequate long term corrosion protection.

It is generally known that a process of organic vapor deposition (OVD),onto a substrate, may be used to provide a thin layer of film, withuniform deposition, no "pin holes," and good edge coverage. Furthermore,such a process does not require the use of solvents, so there is no VOCproblem. However, most of the previous work in the area of vapordeposition has been restricted to small objects. In particular, organicvapor deposition has been used for coating micro-electronic orelectrical components in electronic and medical devices.

A widely used coating or polymeric material formed by organic vapordeposition is poly-p-xylylene polymer (PARYLENE, commercially availablefrom Union Carbide, Hartford, Conn.). For example, U.S. Pat. No.3,342,754 discloses vapor deposition of poly-p-xylylene polymers throughthe cracking of para-xylylene dimers (di-para-xylylene and derivatives)under low pressure. Another patent, JO 1168859A, discloses organic vapordeposition of such a polymer for improving the wear resistance ofsliding plastic parts and for imparting corrosion resistance to metalsurfaces. However, the latter patent differs from the present processwith respect to treatment of the metal surface before and after organicvapor deposition (OVD).

U.S. Pat. No. 4,784,881 discloses organic vapor deposition usingorganophosphates as an adhesion promoter to improve adhesion between thedeposited film and the substrate. However, this patent does not disclosethe use of a metal phosphate, with or without a passivating treatment.Since the patent teaches organic vapor deposition for the purpose ofadhesion improvement of substrates used in the electronic industry, itis not believed that such treatment would provide adequate corrosionprotection to meet automotive standard requirements. To applicants'knowledge, vapor deposition for corrosion protection of automobiles andstructural parts thereof has never been commercially realized.

U.S. Pat. No. 4,950,365 discloses coating poly-p-xylylene over a hardwear-resistant coating such as used in steel tools or instruments. Thefirst step of the two step process disclosed in this patent requires thecoating of a metal substrate with a thin, hard layer of a metal compoundsuch as titanium nitride. The second step consists of applying a uniformconformal material exemplified by poly-p-xylylene.

U.S. Pat. Nos. 4,495,889 and 4,518,623 disclose a method and apparatusfor organic vapor deposition coating, in which system pressure (relatedto the film properties) and evaporation temperature (related to thepolymerization rate) can be continuously monitored and automaticallyregulated so that coatings with desired physical properties anddeposition rate may be obtained.

Plasma surface modification of a coating formed by organic vapordeposition has also been disclosed. For example, U.S. Pat. No. 4,123,308discloses a process wherein a low temperature plasma is employed tochemically modify the surface of a poly-p-xylylene film to incorporateoxygen atoms into the backbone of the polymer. Thereafter, athermosetting resin such as methyl silicone, methyl phenyl silicone,epoxy, polyurethane, or the like may be chemically bonded to thepoly-p-xylylene coated substrate via reaction between oxygen-containinggroups at the surface of the poly-p-xylylene and a reactant component ofthe thermosetting resin.

Notwithstanding the teachings in the prior art, there is a need for amethod for providing improved corrosion protection of metal substratessuch as employed in automotive production, particularly metal substrateswhich will thereafter receive one or more coatings to obtain anaesthetically appealing finish. The application of a protective layer orfilm by organic vapor deposition, for the corrosion protection of metalsubstrates, must result in good adhesion, good edge coverage, and goodbarrier properties.

SUMMARY OF THE INVENTION

It has been discovered that improved corrosion resistance of a metalsubstrate may be realized by the following procedure:

(a) treating the metal substrate with a metal phosphate-containingagent;

(b) applying, by means of organic vapor deposition, a thin film of apolymer directly onto the phosphated surface of said metal substrate;and

(c) subsequently coating said metal substrate with a primer or topcoat.

In a preferred embodiment, the method of the present invention involvescoating a galvanized steel substrate as follows:

(a) treating the metallic substrate with a phosphate-containing agentselected from the group consisting of zinc phosphate or iron phosphate,with or without passivating treatment;

(b) applying, by means of organic vapor deposition, a thin film of apolymer directly onto the phosphated surface of said metallic substrateso treated, wherein said polymer is selected from the group consistingof poly-p-xylylene, poly-chloro-para-xylylene, or a derivative thereof,wherein said organic vapor deposition is carried out under vacuum andthe polymer is formed from a corresponding reactive monomer speciesproduced through pyrolysis of its corresponding dimer; and

(c) subsequently coating said metal substrate with a primer or topcoat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a vapor deposition system according tothe present invention, showing, from right to left, a sublimation tube,a pyrolysis tube, a vacuum chamber, related piping and so forth.

The drawing may be better understood upon reference to the followingdetailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, the present invention is directed to a coatingsystem involving vapor deposition of a polymeric film to protect a metalsurface from corrosion. Such a system is applicable to a variety ofdifferent types of metal in a variety of different sizes and shapes. Forexample, suitable shapes include preassembled autobody, autobody parts,rolls, coils, sheets and so forth. Suitable metal substrates includecold-rolled steel, stainless steel, aluminum, tin, copper, or alloysthereof. However, it was found that the least corrosion occurred whenthe present invention was used to treat galvanized steel as compared tosteel having no zinc protective layer.

Applicants' novel system involves coating a metal substrate with a thinfilm layer or layers of an organic polymer, by organic vapor depositionin an evacuated chamber. After the deposition of such a film, anoptional further step is post-treatment of the surface of the film toenhance adhesion between the film and a subsequent coating, whetherprimer, primer surfacer, or topcoat, to be applied over it. Thispost-treatment of the deposited film layer may be employed to createfunctional groups such as hydroxyl, acid or base which may then react(or be compatible) with the subsequent coating. Such a post-treatmentstep may typically involve reactive non-polymerizing plasma gas such asoxygen, water, or ammonia, or inert plasma gas such as helium or argon.Alternatively or additionally, a reactive polymerizing gas such astrimethylsilane, methane, tetramethyltin, or the like, may be employedfor plasma deposition of a thin film which will provide improvedadhesion to a primer or topcoat.

The substrate to be treated according to the present invention istreated with a metal phosphate containing agent to prepare the surfaceprior to organic vapor deposition (OVD). The metal phosphate containingagent is applied in an effective amount to provide corrosion protectionand promote adhesion of the OVD film to the metal surface. It isbelieved that adhesion is promoted by the microcrystalline surfacecreated by the metal phosphate. Preferred metal phosphates include zincphosphate and iron phosphate.

Metal phosphating of steel, for corrosion protection, is generally wellknown in the automotive finish industry. For example, zinc phosphatecoatings may be made to crystallize from solution onto the metal surfacebeing treated. Typically, a soluble dihydrogen phosphate Zn(H₂ PO₄)₂ ismade to disproportionate to form insoluble tertiary zinc phosphate Zn₃(PO₄)₂ and phosphoric acid. Following treatment of a metal substratewith a metal phosphate, the treated metal substrate may optionally besubjected to passivation. A common passivating agent is chromic acid.

As indicated above, the use of galvanized steel, as a substrate,produces superior results. Galvanizing of the substrate may beaccomplished by vapor deposition of zinc or zinc alloys. Suitablemethods of vapor deposition include evaporation deposition, glowdischarge sputtering deposition, and ion plating. Zinc or Zinc alloyfilms of high quality, having excellent adhesion, can be obtained.Corrosion or mechanical properties of the zinc layer, such as grainstructure, grain size, crystallinity, morphology, and impurity content,may be manipulated in a controllable manner. Such control may beexercised by adjusting the operating parameters such as plasmaconditions, gas flow, substrate temperature, etc.

The metal substrate to be treated according to the present inventionpreferably is galvanized steel coated with a film of zinc or zinc alloyin an amount of 5-120 grams/m², preferably 10-70 grams/m². A significantadvantage of the present invention is that protection of the substrateby the organic vapor deposited coating may be used to reduce thethickness of the zinc layer, used in galvanizing steel, from its currentlevel, for example 60 g/mhu 2, to a significantly lower level, forexample 20 g/m², while obtaining the same degree of corrosionprotection. Thus, the greater the thickness of the OVD film, the lesszinc may be required and vice versa. Important benefits which may bederived from the use of low zinc steel include lower material cost andeasier recycling.

Following treatment with a metal phosphate, the metal substrate thenreceives, by means of organic vapor deposition, a thin film layer of apolymeric material. Preferred polymers are poly-p-xylylenes such aspoly(chloro-p-xylylene), also referred to as PARALYENE C (Union CarbideCorp.). A dense polymer film of a poly-p-xylylene polymer may be formedunder vacuum from a corresponding reactive monomer species producedthrough pyrolysis of its corresponding dimer having been sublimed invacuum. The poly-p-xylylene film may be deposited conformally, in amicroscopic view, on the substrate surface when the reactive monomerspecies is exposed to the surface at a sufficiently low temperature(lower than the ceiling temperature of the polymer). By such method, thepoly-p-xylylene polymer thin film can penetrate to a small area in amicroscopic enclosure such as pores in the phosphate crystal structureon a steel surface.

The commercially available monochloro-substituted dimer, employed toproduce a poly(chloro-p-xylylene) film, has the following chemicalstructure: ##STR1##

Various other kinds of substitutions are possible, as will be known tothose skilled in the art. For example, it is possible to employ thedichloro-para-xylylene dimer. A poly-p-xylylene polymer, employed in thepresent method, may be represented by the following general chemicalstructure: ##STR2##

Substituents R₁ to R₈ may be, for example allyl, aryl, alkenyl, cyano,carboxyl, alkoxy, hydroxy, hydrogen, halogen, and/or amino, andcombinations thereof. Preferred groups are lower alkyls of one to 10carbon atoms, most preferably 1 to 6 carbon atoms, halogens (chlorine,bromine, iodine, and fluorine), and hydrogen. Suitable arylhydrocarbons, having 1-2 benzene rings, include phenyl, naphthyl andderivatives thereof.

Such dimer molecules, when sublimated and subjected to cracking atelevated temperatures under vacuum, are believed to dissociate into thefollowing quinoid form, in the case of the unsubstituted dimer: ##STR3##

The process of polymerization to obtain poly-p-xylylene is well knownand discussed in Yasuda, H., PLASMA POLYMERIZATION, Academic Press(1985) at pages 65-71. See also KIRK-OTHMER ENCYCLOPEDIA OF TECHNOLOGY,VOl. 24, "Xylylene Polymers" at page 744. Methods of preparing dimerssuch as di-p-xylylene or (2,2)-paracyclophane and derivatives, for usein the present invention, have been published. Recent patents, directedto improving yields or other features in synthesizing such dimers,include U.S. Pat. Nos. 3,342,754; 4,532,369; 4,675,462; 4,734,533; andU.S. Pat. No. 4,783,561, hereby incorporated by reference.

The vapor deposition of such polymers as poly(chloro-para-xylylene) canbe carried out in a deposition system depicted in FIG. 1. FIG. 1 shows asublimation tube (11), pyrolysis tube (12), deposition chamber (13),cold trap (14), substrate (15), pressure gauge (16), vacuum pump (17),sublimation heater (18), and pyrolysis furnace (19).

In operation, the substrate (15) is positioned in the center part of thedeposition chamber (13). Meanwhile, a starting material, such as thedimer di(chloro-para-xylylene), is weighed and charged into thesublimation tube (11). Before introducing the starting material, thewhole system is evacuated using the rotary pump (17) to a preselectedvacuum pressure in the range of about 0.001 to 10 torr, typically lowerthan 6 millitorrs, which pressure is measured by the pressure gauge(16). When the temperature in the pyrolysis tube (12) reaches thedesired temperature, suitably 650° C. using furnace (19), the cold trap(14) is filled with liquid nitrogen and the sublimation heater (18) isturned on. The temperature in the sublimation tube (11) is maintained atan elevated temperature, suitably 140° C., and the temperature in thepyrolysis tube (12) is maintained throughout the deposition process.

The substrate temperature is suitably maintained at a temperature from-196° C. to 80° C., preferably -80° to 50° C., and the deposition timeis suitably from 1 minute to 4 hours. The film thickness suitably rangesfrom about 1 to 100 μm, preferably 0.1 to 1.0 mil (2.5 to 25 μm).

After completion of the deposition, the liquid nitrogen is removed fromthe cold trap (14). When the cold trap (14) reaches room temperature,air is released to break the vacuum of the system. The coated substratemay then be removed from the deposition chamber (13).

In terms of the desired properties of the polymeric thin film formedthereby, the preferred deposition vapor is chloro-p-xylylene. A carriergas for the deposition vapor may also be used. The carrier gas may be aninert gas such as argon or helium.

As indicated above, an optional further step in applicants' process ispost-treatment of the surface of the deposited film to enhance theadhesion between the film layer and a subsequent coating, which mayinclude a primer or topcoat. However, in certain cases, for economicreasons (i.e. shorter processing time for higher productivity), it maybe desirable to omit such a post-treatment step of the organic vapordeposited film layer and proceed directly to the coating step.

The post-treatment step may involve a plasma treatment withnonpolymerizing reactive gases such as oxygen, water, carbon dioxide, orammonia, or mixtures thereof, with or without inert gas. Suchpost-treatment of the film layer makes the surface polar by generatinghydroxy groups, acid groups, or base groups (e.g., amines, alkyl amines,amide, and so forth). Alternatively or additionally, post-treatment ofthe OVD film may involve plasma deposition in which a thin organic filmis deposited by means of a polymerizing gas or vapor of substituted orunsubstituted hydrocarbons, organosilanes, or organometallic compounds,the latter containing oxygen, fluorine, phosphorous, zinc, titanium,antimony, aluminum, zirconium, tin, chromium, germanium and/or mixturesthereof. Suitable polymerizing gases include trimethylsilane (TMS),dimethylsilane, hexamethyldisilane, trimethylethoxysilane,methyltrimethoxysilane, hexamethyldisiloxane, methane, ethylene,acetylene, butadiene, acrylic acid, tetrafluoroethylene,trimethylphosphine, trimethylphosphate, trimethylphosphite,trifluorosilane, tetramethyltin, trimethylaluminum, tetraethylgermanium, tetrabutyltitanate, tetramethyltin, or mixtures thereof, withor without inert gas. An example of post-treatment by a plasma processis shown in commonly assigned patents U.S. Pat. No. 4,980,196 and U.S.Pat. No. 4,981,713, hereby incorporated by reference. It may also bepossible to generate polar groups on the surface of the film by chemicalmethods, dry or wet, which methods will be obvious to one skilled in theart. For example, it is possible to treat the surface of the film withchromic acid in order to generate hydroxy or acid groups on the filmsurface, or with SO₃ gas to sulfonate the surface. (See PolymerInterface and Adhesion, published in 1982 by Marcel Dekker, Inc. at page284, section 9.1.2.).

A metal substrate with an OVD film, that is a thin film formed byorganic vapor deposition, has superior corrosion protection compared tothe same metal without the thin film. However, corrosion protection iseven more dramatically improved by application of a subsequent primercoating. Likewise, a metal substrate with both an OVD film and a primercoating has superior corrosion protection to a metal substrate with onlythe primer coating. Alternatively, a topcoat may be applied directlyonto the thin film without a primer, although a primer coating isrecommended for maximum corrosion protection.

A primer may be applied to the OVD film or to a post-treated film, forexample after the plasma deposition of TMS, in any of a number ofdifferent manners well known in the art (e.g., cathodic or anodicelectrocoating, supercritical fluid coating, dipping, spraying, rolling,powder coatings, vacuum deposition polymerization, and so forth).Furthermore any of a number of different primer coatings well known inthe art may be used Examples include, but are not restricted to acrylicsilane, polyester silane, polyester urethane-silane, melamine/polyester,melamine/polyester urethane, epoxy/anhydride, epoxy/ amine, polyesterisocyanate, polyether vinyl, and the like. A preferred primer coating isone which is reactive with the OVD film or post-treated film orintermediate coatings. It is important that a primer have good adhesion,barrier and anti-corrosion properties.

The primer may or may not contain catalysts (or accelerators), such asdialkyl tin oxide compounds, H₂ O, acids, organotitanates, ororganozirconates.

The primer thickness may vary widely. Primer films of 2.5 microns to 125microns thick can be coated on steels, but a preferred thickness rangeis 10.0 microns to 50 microns.

After deposition of a primer, a subsequent coating or topcoat may alsobe applied. The term "topcoat" is herein used generically to includebasecoats, primer surfacers, monocoats, basecoat/clearcoat systems, orany other topcoat system known in the art.

The process according to the present invention is particularlyapplicable to corrosion protection of coil steel that will be employedin automotive manufacture. In a preferred embodiment of a processaccording to the present invention, coil steel, preferablyelectrogalvanized, may be stamped prior to treatment with a metalphosphate and organic vapor deposition. In this embodiment, typicallyboth sides of the stamped part may receive a thin film layer by organicvapor deposition, followed by a subsequent plasma deposition, forexample of trimethylsilane, followed by a primer, preferably a blocksilane polymer, followed by a primer surfacer and a topcoat.

EXAMPLES

All of the following examples, unless otherwise noted, were run asgenerally described in detailed description. The materials employed wereas follows:

(1) The steel substrates were size (4"×6"×0.032"), which was used asreceived.

(2) The deposition chamber was Pyrex™ cross of 6" inside diameter

(3) The cold-trap chamber was Pyrex™ tee of inside diameter.

(4) The sublimation tube was a Pyrex™ tube of 1.5" diameter and 7" long.

(5) The pyrolysis tube was a quartz tube of 1" diameter and 3 feet long.

(6) The pyrolysis furnace, sublimation heater, and temperaturecontrollers were obtained from Hevi-duty Electric Company.

(7) The pressure gauge was a capacitance barometer (available from MKSInstruments as model 220BA).

(8) The vacuum pump was a mechanical rotary pump (available fromSergent-Welch Scientific Company as Model 1375).

(9) The measurement of film thickness was by means of an Elcometer(available from Paul N. Gardner Co., Inc., as model Elcometer 245F).

The examples below involve a corrosion resistance test (scab test), inwhich test panels are scribed to expose the steel to the testingenvironments. The scribe line is at the center of the panel and is about3 inches long. These scribed panels are then subjected to the followingtest cycle:

Monday through Friday:

15 minute immersion in 5% NaCl solution.

75 minute drying in air at room temperature.

22 hour and 30 minute exposure at 85% relative humidity (R.H.) and 60°C. environment.

Saturday and Sunday:

Samples remain in humidity cabinet (85% R.H., 60° C.).

Samples were examined occasionally. After completion of the scabcorrosion test, the test panels were removed from the chamber and rinsedwith warm water. The samples were examined visually for failure such ascorrosion, lifting, peeling, adhesion loss, or blistering. To evaluatethe scribe line corrosion creepback (loss of adhesion between coatingand steel), the distance between the scribe line and the unaffectedcoating was measured: The average of multiple measurements werecalculated.

EXAMPLE 1

A sample was prepared according to the present process tested asfollows. A zinc-phosphated chromic acid rinsed galvanized steelsubstrate was obtained from ACT Corporation as product designation GMC90E Electro Glv70/70; C158 C20 DIW. For organic vapor deposition ofParylene™ C (Union Carbide Corp.) polymer, the mass of dimer ordi(chloro-para-xylylene) was 20 grams, which resulted in a polymer filmof 0.4 mil (10 μm) thick. The film thickness was found to be linearlyrelated to the mass of dimer charged. The deposition time was about 45minutes (from the onset of power on the sublimation heater to thecomplete consumption of the dimer charged).

The sample was then subjected to the corrosion test described above for5 weeks (25 cycles). The dry/wet adhesion was good based on a tape test(ASTM D3359). The average creep distance was 0.7 millimeter, whereas thecreep distance for the electrocoated (1.0 mil thick) control sample was1.0 millimeter. No blistering and little edge corrosion were observed,even if the edge was cut before applying the coating, while in theelectrocoated control sample blisterings and minor corrosion occurred atedges.

EXAMPLE 2

The sample tested was a zinc-phosphated chromic acid rinsed galvanizedsteel substrate with a Parylene™ C coating (0.8 mil). The substrate wasa zinc-phosphated chromic acid rinsed galvanized steel obtained from ACTCorporation as product designation GMC 90E Electro Glv70/70; C168 C20DIW. The deposition of Parylene™ C polymer involved coating thesubstrate twice to obtain a Parylene™ C film of 0.8 mil (20 μm) thick.In each deposition, the mass of dimer charged was 20 grams. After thefirst deposition, the vacuum needed to be broken to recharge the dimerfor the second deposition.

The sample was then subjected to the corrosion test described above for10 weeks (50 cycles). The dry/wet adhesion was good based on a tape test(ASTM D3359). The average creep distance was 0.7 millimeter, whereas thecreep distance for the electrocoated (1.0 mil thick) control sample was1.6 millimeter. No blistering and little edge corrosion were observedeven if the edge was cut before applying the coating, while in theelectrocoated control sample blisterings and corrosion occurred atedges.

EXAMPLE 3

The sample tested was a zinc-phosphated chromic acid rinsed steelsubstrate with a Parylene C coating (0.4 mil). The substrate was azinc-phosphated chromic acid rinsed steel (available from ACTCorporation as product designation GMC-92C; C168 C68 C20 DIW). Thedeposition of Parylene C polymer involved applying a mass of dimer ordi(chloro-para-xylylene) in the amount of 20 grams, which resulted in apolymer film of 0.4 mil (10 μm) thickness. Deposition time was about 45minutes (from the onset of power on the sublimation heater to thecomplete consumption of the dimer charged).

The sample was then subjected to the corrosion test described above for3 weeks (15 cycles). The dry adhesion was good based on a tape test(ASTM D3359), but the wet adhesion was not as good as the phosphatedgalvanized steel. The film lost most of its adhesion after cyclic testdue to the dissolution of phosphated layer which was confirmed bySEM/EDAX analysis.

EXAMPLE 4

The sample tested was a zinc-phosphated chromic acid rinsed steelsubstrate (available from ACT Corporation as product designationGMC-92-C; C168, C20, DIW) with a poly-chloro-para-xylylene coating (0.2mil). This sample was then coated with a thin layer of trimethylsilane(TMS) plasma polymer (150 nm), followed by a silane primer coating(0.5-0.7 mil). The deposition of poly-chloro-para-xylylene involvedapplying a mass of di(chloro-para-xylylene) in the amount of 10 grams,which resulted in a polymer film of 0.2 mil (5 μm) thickness. Thedeposition time was about 20 minutes.

The plasma deposition of poly(trimethylsilane) using trimethylsilane(TMS) vapor was carried out at an audio frequency (40 kHz) power of 50watts and 200-250 volts. The TMS gas flow rate was 3.0 standard cubiccentimeter per minute (sccm). The system pressure was 100 millitorrs andthe power duration was 5 minutes.

To apply a primer coating, the substrate was removed from the chamberand the primer was applied. The primer comprised a resin havingmethoxysilane functional groups. This resin is described in KanegafuchiU.S. Pat. No. 4,801,658, herein incorporated by reference. The primerwas applied to a thickness of 0.5-0.7 mil by dipping the substrate intothe primer. It was then cured at 200° F. for 30 minutes.

An adhesion test was performed on the sample according to a standardtape test (ASTM D3359). The dry and wet adhesion of the silane primercoating to the TMS-treated poly-chloro-para-xylylene were both good,while the adhesions were comparatively poor without the TMS plasmatreatment. Wet adhesion was performed after exposing the sample to anenvironment of 100% relative humidity and 50 ° C. for 96 hours. Thesample was also subjected to a scab test for 5 weeks. After the 5 weektest, the silane coating still retained its good adhesion The averagecreep distance was 0.7 millimeter and there was no blistering. The edgecorrosion protection was very good.

EXAMPLE 5

The sample tested was a zinc-phosphated chromic acid rinsed steelsubstrate (available from ACT Corporation as product designationGMC-92-C; C168, C20, DIW) with a poly-chloro-para-xylylene coating (0.2mil). This sample was then coated with a thin layer of methane plasmapolymer (30 nm), followed by a melamine-polyester primer coating(0.5-0.7 mil). The deposition of poly-chloro-para-xylylene involvedapplying a mass of di(chloro-para-xylylene) in the amount of 10 grams,which resulted in a polymer film of 0.2 mil (5 μm) thickness. Thedeposition time was about 20 minutes.

The deposition of methane plasma polymer using methane gas was carriedout at an audio frequency (40 kHz) power of 50 watts and 200-250 volts.The gas flow rate was 3.0 standard cubic centimeter per minute (sccm).The system pressure was 100 millitorrs and the power duration was 5minutes.

For application of a primer coating, the substrate was removed from thechamber and the primer was applied. The primer was a light gray primercontaining melamine, polyester, and bisphenol-epichlorohydrin typepolymer resins. The primer was applied to a thickness of 0.5-0.7 mil bydipping the substrate into the primer. It was then cured at 290 ° F. for20 minutes.

An adhesion test was performed on the sample according to a standardtape test (ASTM D3359). The dry and wet adhesion of primer coating tothe methane plasma-treated poly-chloro-para-xylylene were both good,while there was little or no adhesion without the methane plasmatreatment. Wet adhesion was performed after exposing the sample to anenvironment of 100% relative humidity and 50 ° C. for 96 hours. Thesample was also subjected to the scab test for 5 weeks. After the 5 weektest, the primer coating still retained its good adhesion. There was noblistering and the edge corrosion protection was very good.

Various modifications, alterations, additions, or substitutions to thepresent invention will be apparent to those skilled in the art withoutdeparting from the scope and spirit of this invention and it will beunderstood that this invention is not unduly limited to the illustrativeembodiments set forth herein.

We claim:
 1. A method for corrosion protection in automotive productionby coating a galvanized metal substrate, which method comprises:(a)treating the galvanized metal substrate with a zinc or iron phosphatecontaining agent; (b) subsequently applying to the phosphated surface ofthe galvanized metal substrate, by means of organic vapor deposition, afilm of polymer; and (c) subsequently coating said film of polymer witha primer and/or topcoat.
 2. The method of claim 1, wherein the zinc oriron phosphate containing agent is applied in an effective amount tocreate a microcrystalline surface.
 3. The method of claim 1, wherein themetal phosphate containing agent comprises zinc phosphate.
 4. The methodof claim 1, wherein the treatment in step (a) comprises applying a zincor iron phosphate containing agent coupled with a passivating rinse. 5.The method of claim 1, wherein the metal substrate is coated with a filmof zinc or zinc alloy in an amount of 5-120 grams/m².
 6. The method ofclaim 5, wherein the metal substrate is coated with a film of zinc orzinc alloy in an amount of 10-70 grams/m².
 7. The method of claim 1,wherein the polymer is poly-p-xylylene or a derivative thereof.
 8. Themethod of claim 7, wherein the poly-p-xylylene ispoly-chloro-para-xylylene.
 9. The method of claim 1, wherein organicvapor deposition occurs under vacuum and said polymer is formed from acorresponding reactive monomer species produced through pyrolysis of itscorresponding dimer having been sublimed in vacuum.
 10. The method ofclaim 9, wherein the pressure for the vacuum ranges from about 0.001 to10 torr.
 11. The method of claim 1, wherein the thickness of the polymerfilm ranges from about 1 to 100 μm.
 12. The process of claim 9, whereinthe deposition time is from 1 minute to 4 hours and the substratetemperature ranges from -196° C. to 80° C.
 13. The process of claim 1,wherein the thin film comprises a plurality of layers deposited insuccession on the metal substrate.
 14. The method of claim 9, whereinthe dimer is a di-para-xylylene or a derivative thereof.
 15. The methodof claim 5, wherein the metal substrate is an automotive body part. 16.The method of claim 1, wherein the primer is selected from the groupconsisting of acrylic silane, polyester silane, polyesterurethane-silane, melamine/polyester, melamine/polyester urethane,epoxy/anhydride, epoxy/amine, and polyester.
 17. The method of claim 1,wherein between steps (b) and (c), the film of a polymer is subjected topost-treatment with a plasma gas in order to enhance adhesion betweenthe film and the subsequent coating.
 18. The method of claim 17, whereinthe post-treatment involves a plasma treatment step with anonpolymerizing gas to polarize the surface of the film.
 19. The methodof claim 18, wherein the nonpolymerizing gas is selected from the groupconsisting of oxygen, water, carbon dioxide, nitrogen, ammonia, andtheir mixtures.
 20. The method of claim 18, wherein the post-treatmentforms hydroxy groups, acid groups or base groups on the surface of thethin film of polymer.
 21. The method of claim 17, wherein thepost-treatment involves a plasma treatment step with a polymerizing gasto deposit an organic film.
 22. The method of claim 21, wherein thepolymerizing gas is selected from the group consisting of anorganosilane, an organometallic compound, and a substituted orunsubstituted hydrocarbon.
 23. The method of claim 22, wherein theorganosilane is selected from the group consisting of trimethylsilane,dimethylsilane, tetramethylsilane, hexamethyldisilane,trimethylethoxysilane, and methyltrimethoxysilane.
 24. The method ofclaim 22, wherein the substituted or unsubstituted hydrocarbon isselected from the group consisting of methane, ethylene, acetylene,butadiene, acrylic acid and tetrafluoroethylene.
 25. The method of claim22, wherein the organometallic compound comprises atoms selected fromthe group consisting of phosphorous, zinc, titanium, antimony, aluminum,zirconium, tin, chromium, germanium and mixtures thereof.
 26. The methodof claim 25, wherein the organometallic compound is selected from thegroup consisting of trimethylphosphine, trimethylphosphate,trimethylphosphite, trimethylfluorosilane, tetramethyltin,trimethylaluminum, tetramethylgermanium, and tetrabutyltitanate.
 27. Amethod for coating a galvanized steel substrate to protect it fromcorrosion, which method comprises:(a) treating the galvanized steelsubstrate with a metal phosphate containing agent selected from thegroup consisting of zinc phosphate and iron phosphate, wherein saidmetal phosphate containing agent is applied in an effective amount tocreate a microcrystalline surface on said substrate; (b) subsequentlyapplying, by means of organic vapor deposition, a film of a polymer ontothe surface of said galvanized steel substrate, wherein said polymer isselected from the group consisting of poly-p-xylylene,poly(chloro-p-xylylene), and derivatives thereof, wherein said organicvapor deposition is carried out under vacuum and the polymer is formedfrom a corresponding reactive monomer species produced through pyrolysisof its corresponding dimer, the latter having been sublimed in vaccuum;(c) subsequently applying a coating to said film of polymer with aprimer and/or topcoat.